During transmission within a cellular communication system, data that is transmitted is error protected in several ways prior to transmission. One form of error protection is convolutional encoding. During encoding, a convolutional encoder encodes input data bits into data symbols at a fixed encoding rate (r) with an encoding algorithm which facilitates subsequent maximum likelihood decoding of the data symbols into data bits (e.g. convolutional or block coding algorithms). For example, a convolutional encoder encodes input data bits (received at a particular bit rate) at a fixed encoding rate of one data bit to n data symbols (rate 1/n) such that the convolutional encoder outputs data symbols at a n*r ksymbol/second rate.
Such a convolutional encoder is illustrated in FIG. 1. The memory of encoder 100 is characterized by its state, which is represented as a v-bit binary number X=xox1 . . . xv−1. For every input bit, the encoder outputs n bits based on the input and v state bits, and then transitions to a next state. More particularly, for every bit that enters the encoder, that bit is stored in the leftmost memory location, and all pre-stored bits are shifted to the right. Two calculations (g0 and g1) are made each bit time by exclusive OR'ing the contents of the specified shift register locations as shown.
Currently 2nd generation Code Division Multiple Access (CDMA) systems utilize a rate ½ convolutional encoding on the downlink, and a rate ¼ convolutional encoding on the uplink, while 3rd Generation systems can utilize ⅓, ¼, or ⅙ rate convolutional encoding. A problem exists when a mobile unit with 2nd Generation and 3rd Generation capabilities (dual mode) is in inter-system soft handoff between systems employing two differing encoding rates. In such a system the combining of multiple forward links cannot be accomplished. This is because different encoder rates of the multiple air interface systems produce a different number of encoded symbols (with the same encoder constraint length K, for example K=9), which cannot be combined. Therefore, a need exists for a method and apparatus for performing soft handoff between systems employing different encoding rates.